How animals cope with the cold

By
Sharon J. Huntington /
January 6, 2004

How does your life change when winter comes? In warm climates, there probably isn't much difference. In places with cold, snowy winters, you probably dress more warmly and maybe go sledding or build snowmen instead of playing baseball or lying on the grass. For animals that live in the wild, though, winter brings big changes. It changes the ways in which they find food and stay warm. Animals have developed many different ways to live through the winter.

Many animals sleep for long hours or days during the winter. Only a few of them truly hibernate, though. True hibernators share three basic characteristics: When they sleep their body temperature drops greatly, sometimes to just a few degrees above the temperature around them. Their breathing slows, and their heart rate slows. The heart rate of the woodchuck drops from 80 beats per minute to just five beats per minute when it hibernates. A hibernating dormouse breathes only once every few minutes.

Some bears and squirrels also hibernate, and all these animals prepare carefully for their winter naps. They find a safe place that will protect them from the weather and from predators. During the late summer and fall they eat large amounts of food. Their bodies convert this food into two types of fat, white and brown. The white fat insulates them from the cold while they sleep. They also live off this fat while they sleep so they won't get hungry. The brown fat gives the animal an energy charge when it's time to wake up in the spring. Some hibernators store up food in their winter nest, so they'll have something to eat when they wake up in the spring.

Many other animals also sleep in winter, but not in the same way true hibernators do. They may become torpid, which means they are very sleepy and don't move very much. Chipmunks sleep most of the winter, but wake up sometimes to eat, So do some squirrels, such as the gray squirrels in Europe and the United States. Like chipmunks, gray squirrels store up food for the times when they wake up hungry. Some bears wake in winter to go out and find food or to give birth to their young.

Snakes and lizards become torpid in winter. They are "cold-blooded" animals that cannot warm themselves well or stay active in cold weather. Garter snakes may cluster together among rocks or in a hole to share body warmth and sleep through the winter. In the spring you may spot a snake or lizard lying still on a rock, letting the sun warm it up after its long rest.

The North American diamondback terrapin buries itself in mud in a pond or lake to wait out the winter. The crayfish also buries itself in mud at the bottom of a pond. Even if the surface freezes, the water near the bottom stays warmer and doesn't freeze. This is where many fish survive the winter, becoming dormant and waiting for spring.

Honeybees pass the winter piled up in a big ball in their hives. They eat the honey they made during the summer. Ants also gather together and eat their stored food, usually in the rooms at the bottom of their anthill, where the ground is warmest.

Some animals don't sleep through the winter and spend a lot of their time finding scarce winter food. Deer eat bark and twigs when there are no leaves or grasses. Mice spend the day in their warm winter nests and come out at night to look for nuts, grains, or crumbs. Beavers spend most of their time inside their houses on a river or stream, eating bark that they stored during the warm months. The American red squirrel buries seeds in autumn, then comes out of its nest in winter to dig them up. Sometimes it forgets where it buried the seeds, and these may grow into new trees. Even some insects, such as the crane fly and snow flea, stay active in winter.

To protect themselves, animals that stay active in winter may change color. The snowshoe hare and weasel are brown in the summer, but in the winter their fur turns white to blend in with the snow. Foxes not only wear white, they also grow extra fur that traps layers of air to provide insulation from the cold. The ptarmigan (TAR-mih-gun) has reddish brown or black feathers in summer and white feathers in the winter. It also has short feathers on its feet that help it walk in the snow.

Many birds and animals don't even try to survive winter climates. They just leave. They migrate to warmer areas to spend the winter. You may have seen large flocks of birds flying overheard, going south for the winter. Some, such as robins, don't have far to fly to reach their warmer winter spots. Others travel long distances. Warblers travel about 2,000 miles. The champion for migrating birds is the Arctic tern. It spends the summer in the Arctic, near the North Pole. Then it flies to the Antarctic, near the South Pole, when the weather grows colder in the Northern Hemisphere. It flies about 11,000 miles - each way.

Reindeer migrate, as do seals, whales, elk, caribou, and some bats, frogs, turtles, and fish. In the Northern Hemisphere the animals migrate south in winter and north in summer. Others migrate up and down: Mountain-dwelling animals such as mule deer and mountain quail live high in the mountains where it is cool in the summer, then come down the mountains to warmer areas in winter. Termites, Japanese beetles, and earthworms migrate downward into warmer soil. Humpback whales find lots of food near the polar regions in summer, but their young could not survive the polar seas in winter, so they swim down to tropical waters closer to the equator to bear their young. Monarch butterflies migrate from Canada and the northern US to as far south as Mexico.

Some insects have the toughest survival habits of all. Butterflies and wasps find quiet places to sleep through the winter, but the praying mantis and cricket don't even try to survive. Instead they lay their eggs in the fall and die. The eggs hatch in the spring. The new insects will live through the summer and lay eggs in the fall. Ladybugs make it through the cold by clustering on twigs and branches. Some insects make a chemical in their bodies that protects them from freezing.

Animals have found lots of ways to survive in the winter. Most of the ways keep them out of our sight. You might be surprised to realize how many animals are still around in winter. They are quietly tucked away in warm nests, waiting for spring.

Before people could travel or communicate easily over long distances, animal and bird migrations seemed mysterious. Animals left the area and birds flew away. No one knew where they went. When animals came back in the spring, were they the same animals that had left in the fall? There was no way to tell.

One way scientists found to track migrating animals and birds was to band them. A small metal band could be placed around a bird's leg or attached to the ear of an animal. If a banded animal from the north was later spotted in the south in the winter, scientists could get a better idea of its movements and those of its herd or flock.

Later, scientists developed tiny electronic devices that could constantly track an animal by sending signals back to researchers. Now even satellites are used to track animals. Scientists can create a map showing where a species travels. They have been astounded at how far some animals travel. Tiny hummingbirds were found to cross the Gulf of Mexico, for example. Some people thought the hummingbirds caught a ride across the long distances on the backs of migrating geese. But now scientists believe the sturdy little hummers get there on their own.

The Internet is a great tool for tracking animal migrations. People across the country or around the world can report sightings of animals that may help us get a picture of their seasonal movements. You can follow some of these studies or even become part of the migration-watching activities yourself.

To learn more about these migration studies, check out these websites:

Students in Canada and the United States can join Canadian astronaut Bob Thirsk in helping to monitor the movements of the eider duck, caribou, peregrine falcon, polar bear, and leatherback sea turtle.